In recent years transluminal angioplasty has become increasingly popular as an effective alternative to coronary bypass surgery. Transluminal angioplasty utilizes an elongated, flexible catheter having a balloon or expandable member at its distal end that is inserted at an appropriate location in the system of a patient. After the catheter is inserted into the vascular system, it is routed through the vascular system to the coronary artery that is partially occluded by a stenotic lesion. Once the catheter is in place and the balloon is properly positioned relative to the lesion, the balloon is inflated by filling the balloon with a fluid that is under relatively high pressure. As the balloon expands, its opens the occluded vessel, thus allowing blood to flow more freely.
When the balloon-tipped catheter is initially removed from its sterile package in preparation for transluminal angioplasty surgery, it is necessary to purge air from inside the catheter prior to inserting the catheter into the patient. All of the air must be removed from the balloon because air compresses under pressure and prevents the balloon from inflating properly. More importantly, when the balloon is inflated with a radiopaque marking liquid, the presence of air in the balloon may result in an error in the accurate positioning of the balloon relative to the lesion being treated. Furthermore, air pockets create artifacts in an image formed by an ultrasonic device, such as that illustrated in U.S. Pat. No. 4,917,097 to Proudian which may be positioned within the balloon.
Conventional methods for purging air from the balloon involve repeatedly filling and aspirating the catheter with a liquid, such as saline solution, so that the air in the catheter tends to be mixed and entrapped in the liquid and drawn out of the catheter with the liquid. The problems associated with this technique have lead to many attempts to design a balloon that is self-purging of air.
Self-purging balloon catheters typically include an air vent that is large enough in diameter to enable air to pass from the balloon, but small enough in diameter to inhibit the flow of liquid from the balloon. While this approach is effective, most configurations of it are relatively costly to produce and test.
For example, conventional self-venting balloon catheters typically incorporate the self-venting feature into the balloon, requiring the balloon to be attached to the catheter before the self-venting feature can be tested. Because the vent is in the balloon, the self-venting feature cannot be tested until after the catheter is fully assembled. Because of the small size of the vents, it is not uncommon for them to not fully form. Because the flaw in the vent is not discoverable until after the catheter is fully assembled, the entire catheter must be discarded. Because the balloon is a relatively expensive component of the balloon catheter, it would be preferable to test the vent before the balloon is mounted to the catheter. Unfortunately, in most configurations of a self-venting balloon, this is not possible.
Furthermore, conventional self-venting balloon catheter configurations, such as those illustrated in U.S. Pat. No. 4,938,220 to Mueller and U.S. Pat. No. 4,638,805 to Powell, inhibit the visibility of bands mounted on the catheters and used as radiopaque markers. The configurations of these patents require a specially machined, split tip marker to be used in order to avoid interfering with the operability of the vent.